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Boost gone missing RB25

OO Dan OO

By OO Dan OO,
in Forced Induction Performance

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OO Dan OO Explorer

OO Dan OO

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My RB25 is lately struggling to get to 5k rpm it feels like it's not boosting. It gets to about 4.5-5k then splutters. It is usually 580hp and would usually scream instantly to 7k rpm in seconds. I have also noticed it is idling really high. Like 1500 rpm which makes me think maybe a vac hose has come off or something. The microtech dash is reading boost but I am not feeling it. If idle is high, would that suggest vac is leaking somewhere after the throttle body?

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OO Dan OO Explorer

OO Dan OO

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When I get a chance I will suss out where everything is connected.

the weird thing is it seemed to start when I put E85 from a different servo than my usual United. My fuel pumps were making a strange high pitch noise for a bit. Not sure if it is coincidence.

btw you seem to reply to every single post in this forum. Are you the administrator?

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GTSBoy Grand Master

GTSBoy

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I'm not an admin here. Just seem to be always in front of a computer.

The different fuel thing is an additional confusion. If it were just a bit short on ethanol content, it really shouldn't cause you to lose boost and make less power. If anything, because it would be more likely to ping, it should make more power - for a little while.

And if it were 98 by accident, it really should ping, although it would be as rich as hell, so there's competing mechanisms there. The MT is not smart enough to do anything about knock, so it should all just happen without any reaction. I'd expect smoke, either way.

A high idle does make it sound like something is open, or just perhaps (and worse), something broken.

How much boost is missing? 16ish psi on the dash. Is that normal?

I have just had a thought. Possibly a fuel related problem could have caused high exhaust temps, could have collapsed the cat. That would slow it down while still allowing boost to show normal.

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OO Dan OO Explorer

OO Dan OO

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The fuel was E85 the only thing that made me wonder if it is a fuel issue was the sound from the fuel pumps. I have had 2 tanks of fuel since. Think 16pounds is normal. Tbh I never even took notice of the boost previously. The engine seems to be really well built judging by the quality of the install and the parts. I will make sure all connections between pipes are tight, I may also chuck a new set of plugs and see if that helps. My idle has always been a bit over the place. 
when cold the idle fluctuates a lot when warm sometimes I will sit at lights at 1000 sometimes it will be 1100 and like yesterday it was a steady 1500. Maybe wherever the leak is is progressively getting bigger 

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joshuaho96 Veteran

joshuaho96

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5 hours ago, OO Dan OO said:

The fuel was E85 the only thing that made me wonder if it is a fuel issue was the sound from the fuel pumps. I have had 2 tanks of fuel since. Think 16pounds is normal. Tbh I never even took notice of the boost previously. The engine seems to be really well built judging by the quality of the install and the parts. I will make sure all connections between pipes are tight, I may also chuck a new set of plugs and see if that helps. My idle has always been a bit over the place. 
when cold the idle fluctuates a lot when warm sometimes I will sit at lights at 1000 sometimes it will be 1100 and like yesterday it was a steady 1500. Maybe wherever the leak is is progressively getting bigger 

Start by searching for a boost leak/vacuum leak. That is the first port of call if your idle is weird.

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GTSBoy Grand Master

GTSBoy

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1 hour ago, OO Dan OO said:

I have heard FG Barra plugs are good but not sure if they will fit the RB

Um. Wat? Just use the correct plugs. It's not like they're in any way difficult to get hold of.

1 hour ago, OO Dan OO said:

Also can anyone ID these Coils?

The blue makes them look like Splitfires. But there have been cheap/shit/nasty technicolour yawn coils out there in the same blue as Splitfire, so how can we tell from that photo? Why don't you pull one out and have a look at it?

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OO Dan OO Explorer

OO Dan OO

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6 hours ago, GTSBoy said:

Um. Wat? Just use the correct plugs. It's not like they're in any way difficult to get hold of.

The blue makes them look like Splitfires. But there have been cheap/shit/nasty technicolour yawn coils out there in the same blue as Splitfire, so how can we tell from that photo? Why don't you pull one out and have a look at it?

I was just looking at the ford ones because a very reputable tuner said they are really efficient for high boost applications. That was for a JZ though. My next move is to swap the plugs n check the coils out.

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OO Dan OO Explorer

OO Dan OO

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Ok so here is where I am at.

i have checked IC connections and boost lines.

changed spark plugs

changed TPS

after changing the plugs it seems to idle better when cold but I still have the same issue. I is really hard to get even to 5 grand it's just holding back big time? What fuel pressure should I be looking for?

 

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MBS206 Grand Master

MBS206

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On 11/12/2023 at 9:37 AM, OO Dan OO said:

Ok so here is where I am at.

i have checked IC connections and boost lines.

changed spark plugs

changed TPS

after changing the plugs it seems to idle better when cold but I still have the same issue. I is really hard to get even to 5 grand it's just holding back big time? What fuel pressure should I be looking for?

 

What plug gap are you set to?

 

Do you have a factory fuel pressure reg, or an aftermarket one?

 

If it's aftermarket, it should be set to what ever pressure it was running when tuned. Typically it'd close to factory pressure, but I'll never trust anyone who is setting up aftermarket parts on a vehicle.

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  • 4 weeks later...
OO Dan OO Explorer

OO Dan OO

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I gapped my plugs to .7, I did find some significant boost leaks. Which are now fixed but... no change from my initial problem. I did find a black mark that could possibly be rogue spark on one coil pack shaft, along with my fuel pumps (2x044) sounding a bit strange.

So to recap 

-new TPS

-new plugs .7

- boost leaks fixed

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Duncan Grand Master

Duncan

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On 30/10/2023 at 8:41 AM, OO Dan OO said:

....the weird thing is it seemed to start when I put E85 from a different servo than my usual United. My fuel pumps were making a strange high pitch noise for a bit. Not sure if it is coincidence.....

Well, the clue was there

13 hours ago, OO Dan OO said:

**fixed**
dirty fuel filter after all that

But no-one guessed it! Internet mechanicing fail 

On the bright side, at least the car was misfiring instead of running lean enough to kill the engine, so that is a win.

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